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RESEARCH ARTICLE

Contrasting Effects of Different Mammalian Herbivores on Sagebrush Plant Communities Kari E. Veblen1*, Kyle C. Nehring1, Christopher M. McGlone2, Mark E. Ritchie3 1 Ecology Center and Department of Wildland Resources, Utah State University, Logan, Utah, United States of America, 2 USDA-ARS Pollinating Insect Research Unit, Utah State University, Logan, Utah, United States of America, 3 Department of Biology, Syracuse University, Syracuse, New York, United States of America * [email protected]

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Abstract

OPEN ACCESS Citation: Veblen KE, Nehring KC, McGlone CM, Ritchie ME (2015) Contrasting Effects of Different Mammalian Herbivores on Sagebrush Plant Communities. PLoS ONE 10(2): e0118016. doi:10.1371/journal.pone.0118016 Academic Editor: Martin Schädler, Helmholtz Centre for Environmental Research (UFZ), GERMANY Received: October 13, 2014 Accepted: January 7, 2015 Published: February 11, 2015 Copyright: This is an open access article, free of all copyright, and may be freely reproduced, distributed, transmitted, modified, built upon, or otherwise used by anyone for any lawful purpose. The work is made available under the Creative Commons CC0 public domain dedication. Data Availability Statement: All relevant data are within the paper and its Supporting Information files. Funding: This work was supported by the Deseret Land and Livestock http://www. deseretlandandlivestock.com/ (MER); Utah Agricultural Experiment Station http://uaes.usu.edu/ (KEV); Utah Agricultural Experiment Station Mineral Lease Funds (MER); USDA-ARS Pollinating Insect Research Unit http://www.ars.usda.gov/main/site_ main.htm?modecode=20-80-05-00(CMM); and the National Science Foundation DEB grants 0087201 and 0244247 (MER). The funders had no role in data collection and analysis, decision to publish, or

Herbivory by both grazing and browsing ungulates shapes the structure and functioning of terrestrial ecosystems worldwide, and both types of herbivory have been implicated in major ecosystem state changes. Despite the ecological consequences of differences in diets and feeding habits among herbivores, studies that experimentally distinguish effects of grazing from spatially co-occurring, but temporally segregated browsing are extremely rare. Here we use a set of long-term exclosures in northern Utah, USA, to determine how domestic grazers vs. wild ungulate herbivores (including browsers and mixed feeders) affect sagebrush-dominated plant communities that historically covered ~62 million ha in North America. We sampled plant community properties and found that after 22 years grazing and browsing elicited perceptible changes in overall plant community composition and distinct responses by individual plant species. In the woody layer of the plant community, release from winter and spring wild ungulate herbivory increased densities of larger Wyoming big sagebrush (Artemisia tridentata, ssp. wyomingensis) at the expense of small sagebrush, while disturbance associated with either cattle or wild ungulate activity alone was sufficient to increase bare ground and reduce cover of biological soil crusts. The perennial bunchgrass, bottlebrush squirretail (Elymus elymoides), responded positively to release from summer cattle grazing, and in turn appeared to competitively suppress another more grazing tolerant perennial grass, Sandberg’s blue grass (Poa secunda). Grazing by domestic cattle also was associated with increased non-native species biomass. Together, these results illustrate that ungulate herbivory has not caused sagebrush plant communities to undergo dramatic state shifts; however clear, herbivore-driven shifts are evident. In a dry, perennial-dominated system where plant community changes can occur very slowly, our results provide insights into potential long-term trajectories of these plant communities under different large herbivore regimes. Our results can be used to guide long-term management strategies for sagebrush systems and improve habitat for endemic wildlife species such as sage-grouse (Centrocercus spp.).

PLOS ONE | DOI:10.1371/journal.pone.0118016 February 11, 2015

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Ungulate Effects on Sagebrush Plant Communities

preparation of the manuscript. With the exception of Deseret Land and Livestock, the funders had no role in study design. Deseret Land and Livestock identified general locations for the exclosures so we could focus the study on known winter ungulate range. However, the specific locations of different treatments were chosen by the authors. Competing Interests: Deseret Land and Livestock provided funding for the building of exclosures. This does not alter the authors’ adherence to PLOS ONE policies on sharing data and materials.

Introduction Large ungulate herbivory shapes the physiognomy and functioning of terrestrial ecosystems worldwide. Both grazing and browsing can profoundly influence the structure and composition of plant communities [1–6] with far-reaching consequences for multiple taxa [7–11], including other ungulate herbivores [12–15]. The presence of grazers, such as cattle, has been implicated in major ecosystem state shifts, including woody plant encroachment and apparently permanent conversions from native to non-native plant communities [16–19] (but see [20–22]). Yet despite the potential for additive or antagonistic effects among different ungulates on plant communities (e.g., [23]), the effects of browsing have been underemphasized in the scientific grazing literature and often times completely ignored. Large herbivores can have contrasting effects on plant community composition and dynamics through their behavior and diet preferences [24, 25], both of which can vary seasonally [26, 27]. Cattle are globally distributed domestic grazers that efficiently digest herbaceous material (i.e., mostly grasses and some forbs). Heavy use by grazers such as cattle can reduce or eliminate perennial grasses, thereby shifting plants towards dominance by woody species (Fig. 1) [28–30] or less palatable herbaceous species [31, 32]. While some wild ungulate grazers (e.g., equids and Bovini) may have effects on plant communities very similar to domestic cattle, other wild ungulates are browsers (e.g., North American mule deer [Odocoileus hemionus]) that feed primarily on woody species and forbs [33] or mixed feeders (e.g., elk [Cervus elaphus]) that both graze and browse and show a variable and broad diet range [34–36]. In highly seasonal systems the feeding behavior of ungulate herbivores varies temporally whereby animals browse woody plants more in winter or spring when herbaceous forage is unavailable [27, 37]. Browsing can exert major controls over woody plant dynamics and may directly oppose the effects of grazing on state changes between woody- and grass-dominated states (Fig. 1) [38–40]. Large ungulate herbivores also have the potential to reduce plant community resistance to disturbance and invasion by undesirable plant species, for instance by preferentially foraging on the most palatable plants, increasing bare ground or damaging biological soil crusts via hoof action [41–44]. The relative roles of different types of large herbivores in invasion dynamics, however, have received little attention in the scientific literature [43]. Rarely are the effects of grazing and browsing herbivores distinguished within a single study [45–50] (but see [51]). An abundance of studies focus on the experimental effects of livestock grazing (e.g., [2, 20, 52–54]). Of those that use controlled replication, most make comparisons between treatments to which livestock grazers do vs. do not have access. Typically wild ungulates are allowed access to both treatment types, thereby controlling for, but not exploring the potentially important role of wild ungulates (which can include grazers, browsers, and/or mixed feeders) in plant community dynamics. Other studies experimentally test the effects of wild ungulates in the absence of domestic grazers [55–59] (but see [15, 54]), a land management scenario which is becoming increasingly uncommon. Experimentally investigating the roles of large herbivores with contrasting feeding habits within a single system in a controlled setting will provide insights into the mechanisms behind plant-herbivore dynamics and management options for multi-use landscapes. Here we use a set of long-term exclosures to determine how cattle (which in our study are grazers that use the site primarily in summer) vs. wild ungulates (which in our study are browsers and mixed feeders that use the study site mainly in winter and spring) affect sagebrushdominated plant communities. These plant communities provide critical ecological services and habitat for endemic plant and wildlife species, but presently cover less than half of their original 60 million ha in the Intermountain West of North America [60–63]. We focus on communities dominated by Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis), for


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Fig 1. Conceptual model. Conceptual model of interactions among browsing, grazing, woody plants, and perennial grasses. Model assumes that wild ungulates (top left) will both browse and graze, while cattle (top right) will primarily graze. Solid arrows indicate direct effects and assume net negative effects of herbivory and competition on plant biomass. Dashed arrows indicate indirect effects of browsing and grazing mediated through competition between woody plants (bottom left) and perennial grasses (bottom right). Browsing and grazing, depending on their timing, intensities, interactions, and net effects, can shift plant communities between woody- and grass-dominated states. doi:10.1371/journal.pone.0118016.g001

which there are no published studies that use controlled experimentation to distinguish domestic grazer and wild ungulate (primarily Rocky mountain elk and mule deer) effects on plant community dynamics. In particular we test how different types of herbivory influence a) shrubs and grasses, the dominant components of the plant community (Fig. 1), and b) non-native species and ground cover (including biological soil crusts) that can further influence plant community dynamics.

Materials and Methods Site Description This study was conducted at Deseret Land and Livestock (DLL), a private ranch headquartered 13 km south of Woodruff, UT, USA (41° 24’ N; 111° 13’ W). Our experimental sites are dominated by Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis) and were


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intentionally located in lower elevation areas that are used by cattle in the spring and summer (May—September) and by wild ungulates predominantly in the winter and early spring (November—March). The primary native ungulates on DLL are elk (Cervus elaphus), mule deer (Odocoileus hemionus), and to a lesser extent, pronghorn (Antilocarpa americana). Both elk and deer migrate to higher elevations during the summer and descend to lower elevations in the winter. Elk are considered mixed feeders, and the proportion of their diets made up of browse increases in the winter, particularly under snowier conditions [27]. Deer, considered browsers, rely heavily on woody material in the winter [27]. The main grass species at the study site are bluebunch wheatgrass (Pseudoroegneria spicata), Indian ricegrass (Achnatherum hymenoides), Sandberg’s bluegrass (Poa secunda), needle and thread (Hesperostipa comata) and bottlebrush squirreltail (Elymus elymoides). The main forbs are spiny phlox (Phlox hoodii), pussytoes (Antennaria spp.), and milk vetch (Astragalus spp.). The primary non-natives are bur buttercup (Ceratocephala testiculata), desert alyssum (Alyssum desertorum), and cheatgrass (Bromus tectorum). Plant species nomenclature and nativity are based on the USDA PLANTS database [64]. Our study focused on three livestock pastures (Dip, Neponset, and Kate) ranging in size from 567–1200 ha. Neponset (1980 m, 6–10% slopes, sandy loam soils) and Kate (2010 m, 4–10% slopes, fine sandy loam soils) pastures are typically grazed on a 5 year rest-rotation schedule. These pastures are grazed by cattle in May or June for three years and in October or November for one year, and then are rested for the fifth year. Dip pasture (2010 m, 6–15% slopes, loam soils) typically is grazed in both the early (May) and late (September) growing season and is rarely rested. The average duration for grazing on each pasture is 10 days to three weeks, depending on forage production and herd size; average cattle herd size per pasture is 1000–1500 head of cattle (Mike Meek, DLL Ranch Manager, Pers. Comm.). Rainfall for the year (July—June) preceding our sample periods was 70.1% of the 20-year average for 2012 and 85.2% for 2013. Rainfall for the 4 months (March—June) preceding our sample periods was 25.7% of the 20-year average for 2012 and 37.8% for 2013.

Study design In 1991 and 1992, we established a set of three 90 m x 90 m plots in each of the three study pastures. Within each pasture, one 90 m x 90 m plot was assigned to each of the following treatment types: 1) Total exclosure (no large ungulates allowed), 2) Cattle exclosure (access by wild ungulates only), and 3) Control (access by wild ungulates and cattle). Total exclosures were constructed with 2.5 m high barbwire fencing (20 strand spaced 20 cm vertically) to fence out all ungulates. Cattle exclosures were constructed with low (1.5 m) barbwire fencing (three strands spaced 50 cm vertically) to allow wild ungulates to traverse the fence; additionally one entire side of each cattle exclosure was opened during the majority of the year (when livestock were not in the pasture) to facilitate wild ungulate access. Wild ungulate use of our study pastures is highest during November—March when wild ungulates focus their foraging activity in these lower elevation areas. Control plots were unfenced. In July of 2012 and 2013, we sampled vegetation, ground cover, and ungulate use along five 50 m transects in the central 60 m x 60 m study area of each 90 m x 90 m plot. The five transects were ten meters apart, oriented perpendicular to the overall slope of the plot. Vegetation frequency data were collected in five evenly spaced 1 m x 1 m quadrats per transect (n = 25 per plot). We recorded densities of live and dead shrubs by height class (